1. Field of the Invention
The invention relates to lubricant and fuel antioxidants. More particularly, it relates to lubricants and fuels to which have been added certain phenol esters of hindered phenyl borates.
2. Discussion of the Prior Art
Various compounds have been developed to combat lubricant and fuel oxidative deterioration. The primary object, of course, was to provide active antioxidants. The next most important object was to discover active boron-containing esters that would also be stable under use conditions. Among the many compounds that have been provided by the art are certain boron esters. U.S. Pat. No. 3,356,707, for example, teaches a class of borate esters, where at least one of the ester groups is a 2,6-dialkylphenol group. The other two ester groups can be additional phenolic residues or simple alkyl, aryl or glycol residues derived from hydroxyl-combining hydrocarbons. The borate esters thus produced are taught to be highly stable under hydrolytic conditions and to have a high degree of antioxidant activity. These are said to overcome the general tendancy of boron esters to hydrolyze and to be destroyed, thus making them unattractive for use in liquid hydrocarbons.
We have found a class of boron esters of hindered phenyl borates, which are only partially hydrolytically stable, and function with a very high degree of antioxidant activity by virtue of that partial but selective hydrolytic instability.